As the global energy crisis is getting worse and worse, the harm caused by gradually drying up of oil resources, air pollution and global warming is aggravating. Governments of the world and automobile enterprises have commonly recognized that energy saving and emission reduction are the trends of future development of automobile technologies. Electric vehicles are becoming more and more recognized by consumers due to characteristics such as zero emission, low noise, low cost of usage and better economy. Meanwhile, the market share of electric vehicles is increasing year by year. The power systems of existing pure electric vehicles mostly adopt a solution in which a single electric motor cooperates with a transmission having a fixed transmission ratio. However, in practical application, this solution has many deficiencies. On one hand, since a large torque is required when the electric vehicle starts up, a transmission having a larger transmission ratio has to be selected. On the other hand, when the electric vehicle is running at a high speed, a small transmission ratio of the transmission is required; otherwise, a drive motor having a high rotational speed has to be selected. When using a transmission having a fixed transmission ratio, it is very difficult to optimize operating points of the motor so that the motor can always operate in an efficient area, thus affecting the driving range of electric vehicle. For a solution of two-gear or multi-gear transmission, in order to realize uninterrupted power, the control in gear-shifting process is relatively complicated. Meanwhile, consumers have higher and higher requirements on the driving range and the power of electric vehicles. Therefore, nowadays, more and more attention are paid to the coupling solution of dual-motor cooperating with transmission in the field.
As a kind of solution, Chinese patent No. CN201510166465.9 discloses a dual-motor electric vehicle device and an electric vehicle equipped with the same, comprising a first motor, a second motor, a planet gear transmission mechanism, a speed-reducing and torque-increasing device, a differential and a controller, wherein the first motor is coupled to a sun gear, the second motor is coupled to a ring gear, and outputs of the first motor and the second motor are output from a planet carrier after being superimposed on the planet gears. When the vehicle speed is smaller than a preset value, the first motor is in a motor mode and rotates in a forward direction, the second motor rotates in a reverse direction and is in a power generation mode so as to charge on-board battery pack; when the vehicle speed is not smaller than the preset value, both the first motor and the second motor are in the motor mode and rotate in the forward direction. This solution is simple in structure and can realize various operation modes. However, it has the following problem: when the vehicle speed is smaller than the preset value, in addition to driving the vehicle, the first motor also has to drive the second motor to generate power, thus making the overall efficiency not high.
As another kind of solution, Chinese patent No. CN201120535844.8 discloses a dual-motor electric vehicle power assembly system and an electric vehicle equipped with the same, comprising a first planet gear transmission mechanism, a second planet gear transmission mechanism, a connection device connecting the first planet gear transmission mechanism and the second planet gear transmission mechanism, a differential connected with the second planet gear transmission mechanism, a first motor connected with an input sun gear in the first planet gear transmission mechanism, and a second motor connected with a ring gear in the first planet gear transmission mechanism. In the electric traction mode, when the torque demand is smaller than a preset value, a second clutch is engaged so that the input ring gear is fixed and torque is input via the first motor; otherwise, the second clutch is disengaged so that the input ring gear is released, and a first clutch is engaged and torque is output via the two motors. In this solution, although a coupling of rotational speeds of two motors can be realized, the mechanical structure using two rows of planet gears for transmission is relatively complicated.